The present invention relates to an arrangement for threading bolts into and out of tapped bores, especially for attaching a lid to and detaching the same from a reactor vessel, such as a nuclear reactor vessel.
When a lid of a reactor vessel is to be attached to or detached from the remainder of the reactor vessel, it is necessary to remove or install the nuts which are threaded on the bolts and together with the latter secure the lid to the vessel proper. Particularly when the reactor vessel is a nuclear reactor vessel of a power plant or the like the removal and installation of the lid would create a health hazard to the operating personnel if they had to be present in the vicinity of the nuclear reactor vessel for the removal or installation of the connecting elements which attach the lid to the remainder of the nuclear reactor vessel. To avoid this danger to the operating personnel, there have been already proposed various devices which are remotely controlled in their operation and which, in sequence, unthread the nuts from the bolts, and thereafter remove the nuts so that the cover can be lifted off the vessel, and which are also capable of performing the above-enumerated operations in the reverse order during the assembly of the lid with the remainder of the vessel.
The arrangements of the above type are satisfactory as far as the avoidance of the health hazard to the operating personnel is concerned. However, it has been found that there are instances where the very presence of the threaded bolts, which continue to project upwardly from the vessel after the nuts have been removed by the remotely controlled arrangement, is disadvantageous. This may, for instance, be the case when the overhead clearance is low and when it is therefore difficult to lift the lid high enough for the bolts to clear the holes provided in the flange of the lid. For this reason it has been proposed not only to remove the nuts, but also to unthread the bolts from the tapped bores in the vessel. This, however, presents special problems because the bolts on a nuclear reactor vessel, in order to be able to withstand the stresses acting thereon, each have a weight of hundreds of pounds. This means that, during the threading of the bolts into or out of the tapped bores in the reactor vessel, a very high frictional resistance is encountered which must be overcome.
To achieve a sequential loosening or even removal of the nuts followed by the removal of the threaded bolts, and the installation of these connecting elements in the reverse order, it has been already proposed, in a copending commonly assigned patent application Ser. No. 792,301, to provide a remotely controlled apparatus including a carrier ring on which there is mounted various equipment which, during the removal of the connecting elements, first tensions the bolts so as to reduce or eliminate the friction between the nut and the lid, then loosens the nut on the threaded bolt, then applies such a force to the bolt as to counteract its own weight, and then unthreads the bolt, together with the nut, from the tapped bore provided in the nuclear reactor vessel proper.
More particularly, the bolt-tensioning device of this copending application is so constructed that it includes a plurality of bolt-carrying units each of which is constituted by a cylinder-and-piston unit, the cylinder of which is rigidly connected to the remainder of the bolt-tensioning device and the piston of which can be detachably connected to the respective threaded bolt. The piston of this unit consists of two coaxial tubes which are shiftable to a limited extent in the axial direction relative to one another, the inner one of these tubes having a multitude of conical bores at its lower end, and a plurality of spheres being respectively accommodated in the bores. On the other hand, the outer one of the tubes has at its lower end a circumferentially complete projection which prevents the spheres from being expelled out of an annular groove of the threaded bolt. To unthread the threaded bolts from the tapped bore of the flange of the vessel, the bolt-tensioning device, which is mounted on a carrier ring, is lowered over the threaded bolts until the carrier ring rests on the flange of the lid. Now, coupling jaws are engaged with corresponding grooves provided at the upper end of the threaded bolt so that afterwards a lifting cylinder-and-piston unit can be energized by a pressurized medium so that the bolt is tensioned and extended. After this extension, the nut threaded on this particular bolt is loosened in that a pinion is introduced through a bore into engagement with the nut and rotates the latter in the required sense. Thereafter, the bolt-tensioning device is lifted and the cylinder-and-piston unit is pressurized so that an upwardly oriented force is exerted on the threaded bolt having such a magnitude as to substantially counteract the weight of the bolt and thus reduce or even eliminate the frictional resistance to the rotation of the bolt in the tapped bore which is attributable to the weight of the bolt. Finally, the threaded bolt is unthreaded from the tapped bore by means of a rotating arrangement which is mounted on a crane, which also carries the bolt-tensioning arrangement, and which revolves the counterbalanced threaded bolt about its axis. As mentioned before, the above-discussed steps are performed in the reverse order during the installation of the lid on the reactor vessel proper.
Experience with this type of an arrangement has shown that, under most circumstances, it performs to satisfaction. However, it has also been established that some problems, mainly concerning the proper alignment of the rotating arrangement with the respective bolt, are still present in this arrangement. Furthermore, this arrangement is not readily capable of removing the nuts alone, or the threaded bolts equipped with such nuts in conjunction with one another.